Recently, the hint of an excess in the electron recoil events observed in the XENON1T experiment that cannot be explained by the previously known background has attracted much attention [1]}. The possibility that the excess event is due to a tritium contribution cannot be ruled out, but various DM candidates have been considered to explain it. Among them, the anomaly-free axion with keV mass is very interesting because it can explain the excess in the XENON1T experiment and at the same time it can explain stellar cooling anomalies [2]}.
